In, for example, a fluid pressure cylinder having a cylinder tube and a piston driven in the cylinder tube, an annular magnet is conventionally attached to an annular groove formed around the piston to detect a driving position of the piston as well as a sensor for detecting the magnetic force of the magnet is attached to the cylinder tube.
This type of annular magnet is attached to the piston by a method of dividing the piston into two portions in an axial direction and fitting the annular magnet into a groove formed between the two portions, or a method of dividing the annular magnet into two semicircular magnets, fitting the two semicircular magnets into an annular groove formed around the outer circumference of the piston from both the sides of it, and bonding the ends of the magnets to each other, or the like.
However, the former method is defective in that since the piston must be divided into the two portions, it is manufactured by a complex process and the cost thereof is increased, whereas the latter method is defective in that a repulsion force is generated from the bonded ends at which the same poles confront with each other, and the bonding job therefore is made more difficult (See Japanese Patent No. 3084074-AO ref. 1).
Further, when a soft magnet is used for the above annular magnet, an annular magnet can be attached to a piston arranged integrally by cutting off the annular magnet at one position, fitting it into an annular groove formed around the outer circumference of the piston while deforming it, and then bonding the end surfaces of the cut-off portions to each other. However, such is the same as the above magnet in that a repulsion force is also generated between the same poles in the end surfaces, and the bonding job therefore is made difficult.
Accordingly, it is desired to simply attach the annular magnet to a piston and the like.